The SP family includes the well characterized maltoporin of E. coli for which the three-dimensional structures with and without its substrate have been obtained by X-ray diffraction. The protein consists of an 18 β-stranded β-barrel in contrast to proteins of the general bacterial porin family (GBP) and the Rhodobacter PorCa Porin (RPP) family which consist of 16 β-stranded &beta-barrels. Although maltoporin contains a wider beta-barrel than the porins of the GBP and RPP families (TC#s 1.B.1 and 1.B.7), it exhibits a narrower channel, showing only 5% of the ionic conductance of the latter porins.